John Calvin Commentary Genesis 18:1

"And Jehovah appeared unto him by the oaks of Mamre, as he sat in the tent door in the heat of the day;" — Genesis 18:1 (ASV)

And the Lord appeared unto him. It is uncertain whether Moses says that God afterwards appeared again to Abraham, or whether, returning to the previous history, he introduces other circumstances here which he had not mentioned before. I prefer, however, the former of these interpretations: namely, that God confirmed his servant's mind with a new vision, just as the faith of the saints requires renewed assistance at intervals.

It is also possible that the promise was repeated for Sarah's sake. What shall we say if, in this manner, he chose to honor the greatness of his grace? For the promise concerning Isaac, from whom redemption and salvation would eventually shine forth to the world, cannot be extolled in terms adequate to its dignity.

Whichever of these views is taken, we perceive that there was sufficient reason why Isaac was again promised. Concerning the word Mamre, we have spoken of this in the thirteenth chapter (Genesis 13:1). Probably a grove of oaks was in that place, and Abraham lived there because of the convenience of the situation.